I'd been having the same issues as many of you with sync and lag-comp. Falling over dead and having an enemy draw in AFTER he killed me, getting apparently one-shot by SMGs, emptying an entire mag into someone only to have them turn around kill me with a single shot, all the things that demonstrate the ineptitude of IW's netcode team. The interesting thing about my situation is that I live immediately adjacent to Timmy 2 Wide, in the same apartment complex, with the same ISP, so we should have the same external network infrastructure. I have a brand new cable modem, a decent router, fully wired connections, and I even have a higher bandwidth cable package than Timmy does. When I connected wirelessly to Timmy's network, most of these problems disappear!
I decided to try to emulate Timmy's connection as best I can. Since my Linksys router has limited QoS options, I downloaded and flashed the dd-wrt freeware firmware. I cranked my uplink and downlink speeds down to match Timmy's, reestablished my port forwarding to open my NAT, and voila, better connection quality to MW3. It's still not perfect: I'm often clearly behind some members of the enemy team, and I still play with rubber bullets occasionally, but now it's one or two timmies instead of the entire enemy team, and I seem to have gotten rid of the Darrian Fawkes kills.
I am still trying to find optimal settings for me, but the weird thing is that I'm seeing decent results at 3M down/1M up. This shouldn't make any practical difference from my usual 30 down/5 up. There is simply no way MW3 can fill either bandwidth, there really shouldn't be any detectable difference for me. I'm going to keep tweaking my settings. If I find any sweet spots, I'll let you know.